As mentioned in this hint, Mountain Lion removed the setting in the Sharing preference pane to turn Web Sharing on and off, even though Apache Web Server is still installed by default. That hint also mentions a third-party preference pane that you can install to toggle Web Sharing in Mountain Lion.

Here’s another solution in the form of an AppleScript. If you copy the script to a .scpt file in ~/Library/Scripts, you can conveniently toggle Web Sharing by selecting the script in the Scripts menu.